1. Field of the Invention
The present invention relates to a power transmission apparatus or so-called transfer apparatus for changing over traveling mode of the vehicle between high speed mode and low speed mode or between 2-wheel drive and 4-wheel drive.
2. Background Art
This kind of the transfer apparatus includes a change-over unit for changing over traveling mode of a vehicle and an example of the change-over unit has been disclosed in, for example, Japanese Examined Patent Publication No.Hei8-14309. This known change-over unit includes a single drum member, which is driven by a motor. First and second camgrooves are formed on an outer circumference of the drum member. On the other hand, first and second shift rods are disposed on both sides of the drum member and the first and second shift rods can reciprocate in the axial direction of the drum member. The first and second shift rods have first and second shift forks for changing the driving mode of the transfer unit respectively and pin type cam followers which engage corresponding cam grooves of the drum member.
In the aforementioned change-over unit, as the drum member rotates, by the function of the first and second cam grooves and cam followers or the cam mechanism, the first and second shift rods namely the first and second shift forks are moved, so that the driving mode of the transfer unit is changed over.
The known change-over unit employs the cam mechanism for converting the rotation of the drum member to movement of the first and second shift forks. Thus, it is necessary to form the first and second cam grooves having a complicated configuration on the outer circumference of the drum member. However, working of these cam grooves is very difficult.
Due to the use of the aforementioned cam mechanism, there is no way but disposing the first and second shift rods in parallel to the axis of the drum member, so that an enlargement of the size of the transfer unit is induced.